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Rotating stall is a complicated phenomenon which may occur when a centrifugal compressor works at low flow rate condition close to the compressor surge limit. Rotating stall and surge may cause a large amount of damage to the compressor components due to instability and high pressure fluctuations which increase vibration of the compressor blades. This research introduces numerical simulations of the rotating stall phenomenon in a high speed centrifugal compressor with vaned and vaneless diffusers. The present study discusses the improvement of two important methods to control stall: the air injection method and the casing grooves treatment method. The method of air injection was optimized by monitoring the effects resulting from the variation of the angle of injection and the injection flow rate. The efficiency of the casing grooves method was related to the grooves' locations, cross section dimensions, and the number of grooves. The current research also presents estimations of the deformation of the compressor blades during the transition stage between stall and surge in order to determine whether the blades will deform safely within the tip clearance gap without touching the casing. Rotating stall simulations showed that the initiation of stall is due to the flow separation and the formation of the tip leakage flow. Simulations showed also that stall development appears at the vaneless area between the impeller and diffuser. Air injection simulations showed that stability is optimized when the air injection angle is approximately double the value of the diffuser inlet vane angle. Simulations showed that in order to achieve effective flow circulation, the aspect ratio of the casing groove cross section should be at least equal to one. Structural analysis showed that the most dangerous region at the deformed blade’s tip surface is located at the transition area between the axial and radial blade profile. Results also showed that the deformation of the blades at the tip region can be limited by changing the shaft bearing stiffness; this is useful to prevent damage to the compressor blades.

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